1. Field of the Invention
The present invention relates to exhaust gas sampling apparatus used in measuring particulate matter (PM), such as soot and soluble organic fractions (SOF), in exhaust gas for quantitative analysis of PM contained in gas exhausted from diesel engines or the like.
2. Description of the Prior Art
One of the techniques for measuring particulate matter (PM) in exhaust gas conforms to the filter weighing method. In this kind of measurement, for example in the partial dilution method, a sampling apparatus of exhaust gas is used in which part of the exhaust gas is split off and fed into a diluting tunnel as sample gas. This sample gas (exhaust gas) is diluted with air in the dilution tunnel, and the diluted exhaust gas is passed through a filter for capturing PM.
In the partial dilution method, the flow rate of sample gas is determined as the difference of the exhaust gas flow rate after diluting with air and the flow rate of the air used in diluting. Typically, a flowmeter for measuring the flow rate of diluting air and a flowmeter for measuring the flow rate of gas after dilution are provided in the measuring passage and diluting air supply passage, with the difference between respective readings from these flowmeter is determined as the sample gas flow rate.
The two flowmeters must be either calibrated periodically at proper intervals or calibrated, if necessary, before the start of measurement. During calibration, as disclosed, for example, in U.S. Pat. No. 5,243,847, an actual sample gas flow rate (i.e., a differential flow rate) is passed as a calibration gas and is measured by a highly precise flowmeter. A coefficient for correcting the measuring error of flow rate between the two flowmeters is determined, and the two flowmeters are corrected.
However, in the method disclosed in the above-mentioned Patent, it is necessary to pass the actual sample gas as calibration gas, and to determine the coefficient for correcting the measuring error of flow rate between the two flowmeters; hence, the calibration procedure is overly complicated.
The present invention is devised in the light of the above situation, and it is hence an object to provide a sampling apparatus for exhaust gas that is capable of calibrating two flowmeters easily and precisely, and that is capable of sampling precisely as desired.
To achieve the object, according to the sampling apparatus for exhaust gas of the invention, a diluting air supply passage for diluting a sampled exhaust gas is connected to an upstream side of a diluting tunnel for introducing a portion of air exhaust gas as a sample gas. The diluting air supply passage has a first flowmeter for measuring flow rate of air. A measuring passage having a filter for capturing particulate matter (PM) in the exhaust gas diluted with the diluting air and a second flowmeter for measuring the flow rate of the diluted exhaust gas flowing through this filter are connected to a downstream side of the diluting tunnel. The sample gas flow rate is determined as the difference of the exhaust gas flow rate after dilution and the flow rate of the diluting air, in which both flowmeters are high in precision.
According to another report of the invention, a passage changeover part is provided at the downstream side of the first flowmeter of the diluting air supply passage, and a passage changeover part is provided at the upstream side of the second flowmeter in the measuring passage. A bypass passage is provided between the two passage changeover parts, and beforehand one flowmeter is calibrating by using this bypass passage and another standard flowmeter which is connected to the flowrate check line, the other flowmeter is calibrated by using the calibrated flowmeter as criterion.
The flowmeters used in the sampling apparatus for exhaust gas are preferably flowmeters of high precision, with a flow rate measuring precision of xc2x10.2% or less, such as Venturi flowmeters.
In the sampling apparatus for exhaust gas of the invention, rather than passing actual sample gas, proper calibration gas is passed into the bypass passage, so that the two flowmeters can be calibrated easily.